Pheromones have become, in recent years, more important in the search for methods of control of pests causing damages to agricultural cultivations. As compared with conventional agents, they have the advantage of an outstanding selectivity for one species only, or for a restricted number of closely related species, without affecting other pest species. It is, therefore, possible to combat a certain pest with the aid of pheromones, without disturbing the ecological equilibrium more than necessary.
Pheromones are secreted outside the insect body and, depending on the type of the reaction they induce, may be divided into aggregating, tracing, sex, alerting pheromones, or others.
The most broadly diffused and most interesting ones due to the applicative possibilities in the control of insect species are the sex pheromones which are secreted more frequently by females (but also by males) and attract the individuals of the opposite sex for copulation.
The natural pheromone mixture is volatile and diffuses in the air also to great distances.
When the pheromone mixture comes in contact with particular sense organs of the males (the chemoreceptive sensilla prevailingly located on the antennae), it attracts them towards the source.
If the pheromone mixture, prepared for example by synthesis, or a mixture endowed with the same effect, is available, it is possible to prepare traps containing the attracting mixture in a suitable formulation capable of ensuring a controlled release thereof. The males of a species, when coming into contact with the mixture spread in the air, are attracted towards the traps, where they are caught or killed.
The utilization of the aforesaid traps permits to carry out massive catches of males of a species, thus drastically reducing the number of copulations and, consequently, the future population of such insect species.
Another useful use of the traps containing sex attractants is that of promoting the monitoring action. In fact, by detecting the number of caught insects in the traps containing the attracting mixture and properly placed in a probably infested area, it is possible to determine with a sufficient exactness the limit of the infested area and the density of population of the insects in such area.
These data enable to intervene, if necessary and where necessary, with the conventional insecticides.
The monitoring action allows therefore to reduce the number of treatments to those strictly necessary and to limit them to the area where the infestation is present, thus attaining obvious economic and environmental advantages and avoiding at the same time the development of insect strains resistant to the insecticide utilized.
Another useful application of an insect sex attracting mixture consists in permeating an infested area with the attracting mixture, during the copulation period. As a consequence thereof, the males of the species are no longer able to distinguish the location of the females and in this way the number of copulations decreases, thus drastically reducing the future population of such insect species in the area.
In all the above-mentioned applications, the main problem to overcome, in a practical utilization, is that of achieving a sufficiently prolonged and constant release of the pheromone in the atmosphere. Since generally the copulation period, which differs from species to species, varies within certain time limits as a function of several not quantifiable environmental and climatic factors, the pheromone mixture introduced by man needs to be present in the air for a time-period sufficiently long as to surely include the short time-period during which copulation of the considered insect species occurs.
The natural pheromones whose composition has been determined contain one or more chemical compounds which may belong to different classes such as, e.g., saturated or unsaturated hydrocarbons, saturated or unsaturated high molecular-weight alcohols, acetates of the aforesaid alcohols, lower esters of fatty acids, aldehydes, ketones, etc.
For a certain number of insect species there are now available formulations of synthetic pheromone mixtures or of substances endowed with an analogous effect, which are employable in agriculture according to the methods illustrated hereinabove.
The problem connected with the slow release has been now rather successfully solved by suitable formulations or by the use of technical devices such as microencapsulation, the use of hollow capillary fibres of polymeric material or of glass, closed at one end, or the incorporation into polymeric matrices.
For the pheromones consisting of (or containing) aldehydes, the problem of the slow release has not yet been conveniently solved, since the aldehyde functional group is highly oxidizable in the presence of sunlight and air. For this reason it is difficult to directly use aldehyde pheromones with efficacious results, for example in traps, because such pheromones oxidize in a short time and are no longer effective in attracting the insects.
The other methods of fight based on the use of aldehyde pheromones present analogous problems.
Some of the most important insect species whose natural pheromone contains aldehydes are indicated in following Table 1.
TABLE I ______________________________________ Insect species in the natural pheromone of which aldehydes are contained. Insect species Aldehydes in the pheromone ______________________________________ Choristoneura fumife- (E)-11-tetradecenal rana Galleria mellenella undecanal Heliothis armigera (Z)-11-hexadecenal + others Heliothis virescens (Z)-11-hexadecenal (Z)-9-tetradecenal Heliothis zea (Z)-11-hexadecenal (Z)-9-hexadecenal (Z)-7-hexadecenal + others Prays citri (Z)-7-hexadecenal Eustrotia candidula (Z)-9-hexedecanal ______________________________________
As is known, aldehydes form adducts with sodium bisulphite and these adducts regenerate aldehyde by treatment with acids or bases ##STR1##
This reaction is used to purify an aldehyde since the bisulphite adducts are often in the crystalline form and can be isolated. By successive treatment with acids or bases the bisulphite ion in equilibrium with the adduct is destroyed, thus obtaining a quick regeneration of the aldehyde.